1. Field of the Invention
This invention relates to error counters for digital transmission systems and, in particular, to an arrangement for determining if the density of errors equals or exceeds the first predetermined number, in which case an alarm indication is provided, or, conversely, to eliminate the alarm indication if the density of errors is less than a second predetermined number.
2. Description of the Prior Art
In a number of prior-art systems which employ digital transmission techniques, the presence of an occasional error is not critical to the recovery of the information contained in the transmitted signal. For example, PCM systems in common use in the United States today employ pulse code groups for each of the 24 channels within the system, each code group containing eight digits. Thus, 192 digits represent the time slots or the total number of digits for each frame. In addition to the code groups, there is a winking framing pulse added at the end of the 192nd digit so that each frame contains 193 time slots with the winking framing pulse occurring in the 193 rd time slot. An occasional error could occur in any one of the pulse code groups or an error could occur in the time slot for the framing digit such that the 1 could be represented as a 0 or the 0 could be represented as a 1 without the occasional errors having a significant deleterious effect on the decoding of the received information. In either case, it is not the occasional error which creates a problem, but it is the repetition of errors occurring within a predetermined time interval which is indicative either of trouble in the transmission medium or in the transmission equipment. Thus, an error density is used to determine at what point in time the number of errors occurring is critical to transmission. Error density is related to the number of errors detected within a predetermined time interval.
As is well known, opposite ends of a digital transmission system are said to be synchronized when the clock frequencies have the same frequency and phase. With respect to PCM systems, it is additionally necessary that they be in step with respect to the digits in the code groups. When the transmitter and receiver are properly synchronized, they are described as being in-frame. When they are not properly synchronized, they are described as being out-of-frame. Thus, identification of the framing information at the receiving end of the system is important, and a means for comparison with respect to what the actual framing condition, i.e., 1 or 0, is supposed to be is of importance. However, it is also important not to restart a reframing cycle on the first detection of an error in the framing comparator. This is discussed in detail on pages 1-15, and in particular pages 14-15, in the article by Davis, D. P., "An Experimental Pulse Code Modulation System for Short-Haul Trunks," Bell System Technical Journal, volume 41, January 1962. FIG. 10 at page 16 shows a simplified drawing of a framing detector using the incoming signal from the received PCM line, and it should be particularly noted that an integrator is used at the output of the framing detector to integrate errors which are recognized by the detector, with a particular level being reached before an out-of-frame condition is indicated.
It is also possible to detect errors in data transmission systems and, in particular, those which employ the duo-binary waveform. An error detection arrangement is disclosed in U.S. Pat. No. 3,303,462. Further, the patent discloses a digital counting arrangement for counting the number of errors which occur and, in addition, the counting arrangement uses a clock reset input whereby the counter is reset to 0 periodically. It is important to note that the clock reset timing is independent of error occurrences, and the error counting arrangement includes no provision for resetting the counter 29 after a predetermined period in which the number of errors in less than a predetermined number.
Another error density detector arrangement is shown in U.S. Pat. No. 2,080,589, issued Mar. 21, 1978, inventor Ralph LeRoy Kline. In that invention the time during which errors are to be counted was initiated by the first error input signal which set a predetermined time interval during which the errors would be counted.